Photo voltaic Photovoltaic cells, commonly known as solar cells, are well known devices for direct conversion of solar radiation into electrical energy.
Generally, solar cells are fabricated on a semiconductor wafer or substrate using semiconductor processing techniques to form a p-n junction near a surface of the substrate. Solar radiation impinging on the surface of the substrate creates charge carriers, electron and hole pairs, in the bulk of the substrate, which migrate to p-doped and n-doped regions in the substrate, thereby generating a voltage differential between the doped regions. The doped regions are coupled to metal contacts on the solar cell to direct an electrical current from the cell to an external circuit coupled thereto.
The efficiency of a solar cell in converting incident radiation or light into electrical energy is directly dependent on the photogeneration rate and the effective lifetime of the carriers within the semiconductor substrate. Consequently, it is generally desirable to measure or characterize these properties, photogeneration rate and effective carrier lifetime, for a wafer or substrate prior to fabrication of a solar cell or cells therein, and, using test wafers, at several points during the fabrication process.
One technique commonly used to measure photogeneration rate and effective carrier lifetime in silicon solar cells involves photoluminescence imaging or mapping of the wafer. Briefly, in photoluminescence imaging a steady-state coherent light source, such as a laser, having a particular wavelength (i.e. 800 nm) or a narrow range of wavelengths, is used to stimulate photogeneration of carriers within the wafer. The density of these carriers can then be estimated by monitoring their radiative recombination using a highly sensitive camera to capture the photoluminescence image. A complex and expensive filtering system is required in front of the camera to filter out the stimulation light.
Accordingly, there is a need for a simplified photoluminescence mapping system and method for use in fabricating solar cells that eliminate the need for complex and expensive light sources, filters and high sensitivity cameras.